SIRIUS (Alpha Canis Majoris). From Orion, look south and to the east to find brilliant
Sirius, as if one really needs directions to find the brightest
star in the sky. Its name comes from the Greek word for "searing"
or "scorching," certainly appropriate for a star that shines at the
bright end of the "minus-first" (-1.47) magnitude. Sirius is the
luminary of the constellation Canis Major,
the Greater Dog, which represents Orion's larger hunting dog, and
as such is commonly referred to as the "Dog Star." So great is its
prominence that it has two "announcer stars" that from the mid-
northern hemisphere rise before it, Procyon and Mirzam. Famed from
times long past, the first glimpse of Sirius in dawn announced the
rising of the Nile in ancient Egypt. (It no longer does because of
precession, the 26,000-year wobble of
the Earth's axis.) Sirius is also part of a large asterism, the Winter Triangle, the other two stars of
which are Betelgeuse in Orion and Procyon in the smaller dog, Canis Minor. Because of its brilliance, Sirius is the
champion of all twinklers, the effect caused by variable refraction
in the Earth's atmosphere. The star, a white class A (A1)
hydrogen-fusing dwarf with a temperature of 9880 Kelvin, is bright
in part because it is indeed rather luminous, 26 times more so than
the Sun, but mostly because it is nearby, a
mere 8.6 light years away, just double that of the closest star to
the Earth (Alpha Centauri) and the
fifth closest star system. Sirius is "metal rich," its iron
content perhaps double that of the Sun, most likely from some sort of
elemental diffusion. With a radius of 1.75 solar (in agreement
with the measured angular diameter) and a minimum equatorial
rotation speed of 16 kilometers per second, Sirius rotates in under
5.5 days. The star's greatest claim to fame may be its dim eighth
magnitude (8.44) companion, Sirius B, which is visually nearly
10,000 times fainter than the bright star, Sirius A. Sirius B,
however, is actually the hotter of the two, a blue-white 24,800
Kelvin. Though typically separated from each other by a few
seconds of arc, Sirius B is terribly difficult to see in the glare
of Sirius A. The only way the companion star can be both hot and
dim is to be small, only 0.92 the size of Earth, the total luminosity
(including its ultraviolet light) just 2.4 percent that of
the Sun. The two orbit
each other with a 50.1 year period at an average distance of 19.8
Astronomical Units, about Uranus's distance from the Sun, a large
orbital eccentricity carrying them from 31.5 AU apart to 8.1 AU and
back again. They were closest in 1994 and will be again in 2044,
while they will be farthest apart in 2019. From the orbit (and
spectroscopic data), we find that Sirius A and B have respective
masses of 2.12 and 1.03 times that of the Sun. Sirius B is the
chief member of a trio of classic white dwarfs, the others Procyon B and 40
Eridani B. Its high mass and tiny radius lead to an amazing
average density of 1.7 metric tons per cubic centimeter, roughly a
sugar cube. White dwarfs are the end products of ordinary stars
like the Sun, tiny remnants that were once nuclear-fusing cores
that have run out of fuel. Most are balls of carbon and oxygen
whose fates are merely to cool forever. To have evolved first,
Sirius B must once have been more massive and luminous than Sirius
A. That its mass is now lower is proof that stars lose
considerable mass as they die. Given the mass of the white
dwarf and the 250 million year age of
the system, Sirius B may once have been a hot class B3-B5 star that
could have contained as much as 5 to 7 solar masses, the star perhaps
losing over 80 percent of itself back into interstellar space
through earlier winds. (Thanks to Steve Ash for prompting a
rewrite.)